Two-dimensional hybrid simulations of the oblique electromagnetic alpha/proton instability in the solar wind

In the solar wind, alpha particles are observed to flow faster than the core protons. In this paper, two-dimensional hybrid simulations are performed to investigate the nonlinear evolution of oblique Alfvén waves excited by an alpha/proton beam instability in a low beta plasma. The propagation angles of the excited waves are within a finite range suggesting the generation of oblique Alfvén waves. During the nonlinear evolution, both the wave numbers and frequencies of the waves drift to smaller values, and the propagation angles decrease. At the same time, the propagation angle of the dominant mode also changes. Eventually the plasma system reaches a marginally stable state according to linear theory.